A level-shift semiconductor device having a so-called divided RESURF (Reduced Surface Field) structure has been hitherto known, for example, as disclosed in Japanese Patent No. 4,574,601. The divided RESURF structure is related to a technique in which a high withstand-voltage level-shift device to which a well-known RESURF structure is applied is formed in a high withstand-voltage isolation region to which a well-known RESURF structure is applied. Since the RESURF structure is a well-known structure as described in Japanese Patent No. 4,574,601, etc., the description thereof is omitted. In the divided RESURF structure disclosed in this patent literature, each of the high withstand-voltage isolation region and the high withstand-voltage level-shift device is formed of an N-type diffusion layer. The high withstand-voltage level-shift device and the high withstand-voltage isolation region are electrically isolated from each other by a P-type isolation region.